1. Field of the Invention
The present invention relates to a stereo microphone, specifically a technology varying the angle between the left and right directional axes, i.e., localization of a microphone used for MS stereo recording.
2. Related Background Art
One of the stereo recording techniques is MS recording, in which sound is recorded separately in a middle (M) direction and a lateral direction or side (S) directions. Microphones for MS stereo recording are commercially available. An MS stereo microphone includes a unidirectional microphone element and a bidirectional microphone element, the unidirectional microphone element picking up sound from the middle direction, the bidirectional microphone element picking up sound from the side directions. Directional axes of the microphone elements are disposed orthogonally.
FIG. 3 illustrates an example of the MS stereo microphone, which includes a middle unidirectional microphone element 10 and a bidirectional side microphone element 20. Condenser microphone elements are used for the microphone elements 10 and 20 in the example. The microphone elements 10 and 20 are mounted in a microphone case 40, more specifically in a mesh cover 42 covering the upper-half portion of the microphone case 40. The microphone element 20 is disposed above the microphone element 10. The microphone elements 10 and 20 are fixed to the microphone case 40 such that directional axes thereof are horizontal and at an angle of 90 degrees with each other. The microphone case 40 is provided with circuits described below. A connector 41 outputting output signals of the microphone to an external device is provided at the bottom of the microphone case 40.
The principle of stereo recording using the MS stereo microphone, including variable localization in MS stereo recording, is schematically explained with reference to FIGS. 8A to 8C. The left drawings in FIGS. 8A and 8B illustrate the directivity of the middle unidirectional microphone element, in which a cardioid curve is drawn as commonly known. The middle drawings in FIGS. 8A and 8B illustrate the directivity of the bidirectional side microphone element. Signs “+” and “−” in the directivity curves represent directions of the sound pressure. In MS stereo recording, the sum of the output of the middle microphone element and the output of the side microphone element yields right (R) channel signals; while the difference between the output of the middle microphone element and the output of the side microphone element yields left (L) channel signals.
The right drawing in FIG. 8A illustrates the right channel signal. Adding the output of the middle microphone element to the output of the side microphone element provides hyper cardioid directivity centering on an axis line inclining approximately 63.4 degrees to the left from the reference axis of the microphone center. The right drawing in FIG. 8B illustrates the left channel signal. Subtracting the output of the side microphone element from the output of the middle microphone element provides hyper cardioid directivity centering on an axis line inclining approximately 63.4 degrees to the right from the reference axis of the microphone center. Thereby, a sound pick-up signal having the directional axis inclining toward the right and a sound pick-up signal having the directional axis inclining toward the left can be produced. The directivity of the two sound pick-up signals is hyper cardioid and symmetric relative to the reference axis of the microphone center. Thus, a stereo signal can be produced from the two sound pick-up signals.
In FIGS. 8A and 8B, the directivity of the right channel sound pick-up signal and the directivity of the left channel sound pick-up signal theoretically incline 63.4 degrees to the right and the left, respectively, from the reference axis of the microphone center. Thus, the angle defined by the directional axes of the left and right channels is approximately 127 degrees, as shown in FIG. 8C. Such an angle of the directional axes of the left and right channels of 127 degrees is applicable to recording under most circumstances. It may be desired, however, that the angle be variable according to preference or a variety of conditions, such as spaciousness of a recorded object, and that commonly called localization thus be variable. In order to change the angle between the directional axes of the left and right channels in MS stereo recording, the output of the side microphone element may be adjusted, which is added to or subtracted from the output of the middle microphone element. Alternatively, the output level of the middle microphone element may be set to be variable, while the output level of the side microphone element may be set to be invariable, in order to change the angle between the left and right directional axes. If the angle between the left and right directional axes is too large or too small, the stereo effect is diminished and localization is unclear. In general, the lower limit of the angle between the left and right directional axes is deemed to be 90 degrees and the upper limit is 127 degrees, as shown in FIG. 8C.
A specific example of a conventional MS stereo microphone is explained below. In FIG. 4, the middle unidirectional microphone element 10 and the bidirectional side microphone element 20 are shown, as described above. The microphone elements 10 and 20, which are condenser microphone elements, are supplied with a polarization voltage from a power circuit 22 including a DC-DC converter. The power circuit 22 boosts a power source battery voltage of approximately 5V to approximately ±100V, and applies the voltage to a diaphragm and an opposed fixed plate of each of the condenser microphone elements. Output signals from the microphone elements 10 and 20 are amplified at amplifiers 11 and 21, respectively, and then output.
The microphone outputs are separated into a left channel and a right channel. For three-pin balanced output of each channel signal, a circuit configuration is provided as below. The output end of the amplifier 11 that amplifies the output of the middle microphone element 10 is connected to a second pin of the left channel through an amplifier 26. The output end of the amplifier 11 is also connected to a second pin of the right channel through an amplifier 27. The output end of the amplifier 21 that amplifies the output of the side microphone element 20 is connected to a third pin of the right channel through an amplifier 29. The output end of the amplifier 21 is also connected to an inverting input terminal of an inverting amplifier 25 that includes a differential amplifier, through an input resistor Rs. A feedback resistor Rf is connected between the output terminal and the inverting input terminal of the inverting amplifier 25. The ratio of the input resistor Rs to the feedback resistor Rf changes a phase difference of the output signal from the inverting amplifier 25. The ratio is set herein at Rs=Rf such that the phase difference between the output signal and the input signal is 180 degrees. The output end of the inverting amplifier 25 is connected to a third pin of the left channel through an amplifier 28. The amplifiers 26 to 29 are each emitter-follower-connected.
If the middle output M from the amplifier 11 has a + phase, an M+ signal is output from each of the second pins of the L channel and the R channels. The second pins are hot output terminals of the balanced output. Meanwhile, the side output S from the amplifier 21 also has a +phase. Then, an S+ signal is output from the third pin of the right channel. The phase of the side output S+ from the amplifier 21 that passes through the inverting amplifier 25 is inverted to S−. The inverted signal S− is output from the third pin of the left channel through the amplifier 28. Both the left channel signal and the right channel signal are output from a three-pin connecter as a balanced signal. First pins of the respective channels are grounded. The second pins are hot signal pins as described above, and the third pins are cold signal pins.
As described above, the signals M+ and S− are balance-output from the left channel L, and the signals M+ and S+ are balance-output from the right channel R. The balanced output of the left channel L, which is composed of the middle output M+ having a + phase on the hot side and the side output S− having a − phase on the cold side, shows the directivity centering on the axis line inclining toward the right from the reference axis, as shown in FIG. 8B. The balanced output of the right channel R, which is composed of the middle output M+ having a + phase on the hot side and the side output S+ having a + phase on the cold side, shows the directivity centering on the axis line inclining toward the left from the reference axis, as shown in FIG. 8A. Thereby, a stereo sound signal is provided.
In the MS stereo microphone described above, it may be required to narrow the sound pick-up angle from 127 degrees toward 90 degrees, for example, in a case of a narrow sound source, for example, as explained with respect to FIG. 8C. FIGS. 5 through 7 illustrate typical MS stereo microphones each having a variable sound pick-up angle.
In FIG. 5, the power circuit including the DC-DC converter is separated into a middle power circuit 221 and a side power circuit 222. For example, the power voltage of the middle power circuit 221 is fixed while the power voltage of the side power circuit 222 is variable. The polarization voltage is thus variable for the side microphone element 20. While the output level of the middle microphone element 10 is constant, the output level of the side microphone element 20, which is added to or subtracted from the output of the middle microphone element 10, can be adjusted by varying the polarization voltage. Consequently, the sound pick-up angle, or the angle of the directional axes of the left and right channels, can be varied.
The two power circuits including DC-DC converters as shown in the example of FIG. 5, however, interfere with each other to generate beat noise. Specifically, in each of the two power circuits 221 and 222, a transformer increases the voltage of an alternating signal generated by oscillation of about 1.2 MHz or switching operation. The voltage is rectified, and then increased from approximately DC 5V to approximately DC±100, for example. Furthermore, self-oscillation circuits are used in the power circuits 221 and 222 for cost reduction purposes. Since oscillation frequencies of the power circuits 221 and 222 are unstable, it is difficult to match the oscillation frequencies. As a result, beat noise is generated because of a difference between oscillation frequencies f1 and f2 of the respective power circuits. In addition, the two power circuits 221 and 222 generate signals at a relatively high frequency as described above. Thus, the two power circuits 221 and 222 easily form a magnetic coupling and easily interfere with each other, thus generating noise. Measures for noise prevention may include use of a crystal oscillator to stabilize the oscillation frequencies of the respective power circuits 221 and 222. This measure is unfavorable, however, since it leads to an increase in microphone production cost.
FIG. 6 illustrates an alternative example in which the polarization voltage is variable for the side microphone element 20. In the example, the polarization voltage for the side microphone element 20 is variable through switching of voltage-dividing resistors. Resistors Rd1, Rd2, Rd3, and Rd4 are connected in series between a +Vp output terminal and a −Vp output terminal of the power circuit 22 that supplies the polarization voltage to the middle microphone element 10 and the side microphone element 20. The output terminal of the power circuit 22 is connected to the middle microphone element 10, and thus the output voltage of the power circuit 22 is directly applied thereto. A switch 31 is provided so as to select either the +Vp output terminal of the power circuit 22 or the node of the resistors Rd1 and Rd2. A switch 32 is further provided so as to select either the −Vp output terminal of the power circuit 22 or the node of the resistors Rd3 and Rd4.
The switches 31 and 32 are operated in conjunction with each other. In a first switch setting, the output voltage of the power circuit 22 is directly applied as the polarization voltage to the side microphone element 20. In a second switch setting, a partial voltage of the resistors Rd1, Rd2, Rd3, and Rd4 is applied as the polarization voltage. In the case where the switches 31 and 32 are set as represented by a solid line in FIG. 6, the polarization voltage for the side microphone element 20 is high, and then the angle between the directional axes of the left and right channels is wide, as explained in the previous example. In the case where the switches 31 and 32 are set as represented by a broken line in FIG. 6, the polarization voltage for the side microphone element 20 is decreased, and then the angle between the directional axes of the left and right channels is narrowed, as explained in the previous example.
In the example of FIG. 6, current flows from the DC-DC converter, which is the main component of the power circuit 22, to the voltage-dividing resistors Rd1, Rd2, Rd3, and Rd4. Thus, the consumption current of the DC-DC converter increases. The DC-DC converter is provided with a rectifier. An increase in consumption current of the DC-DC converter increases a voltage drop of the rectifier, and thus decreases the output voltage. In order to prevent this, the resistance value of the voltage-dividing resistors as a whole is set to be a high value, such as, for example, 1 MΩ to minimize the current flowing to the voltage-dividing resistors. It is unavoidable, however, that the current flows to the voltage-dividing resistors. Thus, the output power of the DC-DC converter is consumed at the voltage-dividing resistors, and the current required for a signal system is limited.
FIG. 7 illustrates a further alternative example in which the angle (localization) between the directional axes of the left and right channels is variable by changing the output level of the side microphone element 20. In the example, voltage-dividing resistors Rd5, Rd6, Rd7, and Rd8 and switches 33 and 34 operating in conjunction with each other are connected to the output circuit of the inverting amplifier 25 which is connected to a side signal circuit. Thereby, the side output level is changed. The voltage-dividing resistors Rd5, Rd6, Rd7, and Rd8 are connected in series between the output end of the amplifier 21 that amplifies the output signal of the side microphone element 20 and the output end of the inverting amplifier 25. The switch 33 is connected so as to select either the output end of the inverting amplifier 25 or the node of the resistors Rd5 and Rd6, and to input the selection to the amplifier 28. The switch 34 is connected so as to select either the output end of the amplifier 21 or the node of the resistors Rd7 and Rd8, and to input the selection to the amplifier 29.
In the case where the conjunction switches 33 and 34 are set as represented by a solid line in FIG. 7, the output from the inverting amplifier 25 is directly input to the amplifier 28 while the output from the amplifier 21 is directly input to the amplifier 29. Thus, the side signals S− and S+ are input at the maximum level to the amplifiers 28 and 29, respectively, and then the angle between the directional axes of the left and right channels is wide. In the case where the conjunction switches 33 and 34 are set as represented by a broken line in FIG. 7, the output S− of the inverting amplifier 25 is voltage-divided at the voltage-dividing resistors and then is input to the amplifier 28; and the output S+ of the amplifier 21 is voltage-divided at the voltage-dividing resistors and then is input to the amplifier 29. Thus, the levels of the side signals S− and S+ input to the amplifiers 28 and 29 are decreased, and then the angle between the directional axes of the left and right channels is narrowed.
The angle between the directional axes of the left and right channels can be changed, as shown in the example of FIG. 7. In the example, however, in the case where the value of the voltage-dividing resistors Rd5, Rd6, Rd7, and Rd8 connected to the output circuit of the side signals is low, the voltage-dividing resistors are large load for the inverting amplifier 25, and thus the output signal from the inverting amplifier 25 is distorted. Increasing the value of the voltage-dividing resistors Rd5, Rd6, Rd7, and Rd8 reduces distortion of the output signal from the inverting amplifier 25. However, the level of resistance noise generated at the voltage-dividing resistors Rd5, Rd6, Rd7, and Rd8 is increased, and the signal-to-noise ratio (S/N) is degraded.
A stereo microphone disclosed in Japanese Unexamined Patent Application Publication No. 2006-174136 is known as a conventional MS stereo microphone. Furthermore, a signal-processing technology, such as coding and decoding of MS stereo signals, is also known (refer to Patent Japanese Unexamined Patent Application Publication Nos. 2008-028574 and 2007-004050, for example). However, the inventions disclosed in these patent literatures cannot change the angle between the directional axes of the left and right channels.
The configurations shown in FIGS. 5 through 7 are candidates for MS stereo microphones capable of changing the angle of the directional axes of the left and right channels. As explained above, however, the examples have problems, such as generation of beat noise from the power circuits; consumption of the output power from the DC-DC converter serving as a main component of the power circuit at the voltage-dividing resistors and limited current required for the signal system; and the voltage-dividing resistors being large load for the inverting amplifier, and distortion of the output signal of the inverting amplifier or noise generation.